Implantable neurostimulation systems have proven therapeutic in a wide variety of diseases and disorders. Spinal Cord Stimulation (SCS) techniques, which directly stimulate the spinal cord tissue of the patient, have long been accepted as a therapeutic modality for the treatment of chronic pain syndromes, and the application of spinal cord stimulation has begun to expand to additional applications such as angina pectoralis and incontinence. In recent investigations, Peripheral Stimulation (PS), which includes Peripheral Nerve Field Stimulation (PNFS) techniques that stimulate nerve tissue directly at the symptomatic site of the disease or disorder (e.g., at the source of pain), and Peripheral Nerve Stimulation (PNS) techniques that directly stimulate bundles of peripheral nerves that may not necessarily be at the symptomatic site of the disease or disorder, has demonstrated efficacy in the treatment of chronic pain syndromes and incontinence, and a number of additional applications are currently under investigation.
These implantable neurostimulation systems typically include one or more electrode carrying stimulation leads, which are implanted at the desired stimulation site, and a neurostimulator implanted remotely from the stimulation site, but coupled either directly to the stimulation lead(s) or indirectly to the stimulation lead(s) via a lead extension. Thus, electrical pulses can be delivered from the neurostimulator to the stimulation electrode(s) to stimulate or activate a volume of tissue, thereby providing the desired efficacious therapy to the patient.
The neurostimulation system may further comprise a handheld patient programmer to remotely instruct the neurostimulator to generate electrical stimulation pulses in accordance with selected stimulation parameters. The handheld programmer may, itself, be programmed by a technician attending the patient, for example, by using a Clinician's Programmer (CP), which typically includes a general purpose computer, such as a laptop, with a programming software package installed thereon.
Individual electrode contacts (the “electrodes”) are arranged in a desired pattern and spacing in order to create an electrode array. The combination of electrodes used to deliver electrical pulses to the targeted tissue constitutes an electrode combination, with the electrodes capable of being selectively programmed to act as anodes (positive), cathodes (negative), or left off (zero). In other words, an electrode combination represents the polarity being positive, negative, or zero. Other parameters that may be controlled or varied include electrical pulse parameters, which may define the pulse amplitude (measured in milliamps or volts depending on whether constant current or constant voltage is supplied to the electrodes), pulse width (measured in microseconds), pulse rate (measured in pulses per second), pulse shape, and burst rate (measured as the stimulation on duration per unit time). Each electrode combination, along with the electrical pulse parameters, can be referred to as a “stimulation parameter set.” The best stimulus parameter set will typically be one that delivers stimulation energy to the volume of tissue that must be stimulated in order to provide the therapeutic benefit (e.g., pain relief), while minimizing the volume of non-target tissue that is stimulated.
Typically, the therapeutic effect for any given neurostimulation application may be optimized by adjusting the stimulation parameters. For example, the volume of activated tissue in any given neurostimulation application may be increased or decreased by adjusting certain stimulation parameters, such as amplitude and pulse width. Often, these therapeutic effects are correlated to the diameter of the nerve fibers that innervate the volume of tissue to be stimulated (i.e., for different stimulation applications, different fiber diameters can encode different sensations).
For example, in PNFS and PNS applications, there is often a distribution of fiber diameters near the electrodes that strongly encode different sensations. For example, the larger Abeta afferent nerve fibers in the periphery can encode vibration and pressure, whereas the smaller Adelta nerve fibers often encode sharp pain. In these applications, if the stimulation amplitude is increased for a fixed electrode combination and pulse width, the activation of the large nerve fibers will be increased prior to the activation of the small nerve fibers due to the inherent nature of fiber diameters and the electrical field external to the nerve fibers. However, the patient may reach an amplitude limit due to the activation of the smaller nerve fibers that generate side effects before the larger nerve fibers that provide the intended therapy. Thus, stimulation of the small diameter nerve fibers may lead to other uncomfortable, painful sensations near the stimulating electrode, thereby producing side effects and limiting therapeutic coverage. Therefore, in certain stimulation applications, control of nerve fiber recruitment based on diameter might be critically important to maximize the therapeutic effect of the stimulation.
In contrast, in SCS applications, activation of different nerve fiber diameters does not necessarily encode different sensations or side effects. In particular, in SCS, activation (i.e., recruitment) of large diameter sensory fibers in the dorsal column of the spinal cord creates a sensation known as paresthesia that can be characterized as an alternative sensation that replaces the pain signals sensed within the affected region of the patient's body. Thus, it has been believed that the large diameter nerve fibers are the major targets for SCS. However, the distribution of sensory nerve fiber diameters in the dorsal column is an artifact of where the fibers enter the spinal cord and the specific spinal cord segment stimulated. Since the nerve fibers in the dorsal column tend to be mostly sensory nerve fibers, it is generally believed that different fiber diameter activation merely would result in more or less paresthesia in different parts of the patient's body—essentially all innocuous, if not therapeutic stimulation. Therefore, in some stimulation applications, control of nerve fiber recruitment based on diameter might not be as critically important to maximize the therapeutic effect of the stimulation.
Thus, there remains a need to selectively provide a means for stimulating a specific range of nerve fiber diameters over a range of different stimulation amplitude levels.